Wireless power receiver

ABSTRACT

A wireless power receiver for wirelessly receiving driving power from a wireless power transmitter and a method for wirelessly receiving charging power at a wireless power receiver from a wireless power transmitter are provided. The wireless power receiver includes a power reception unit configured to wirelessly receive the charging power from the wireless power transmitter; a rectifier configured to rectify the charging power from the power reception unit; a load unit configured to store the rectified charging power from the rectifier; and a controller configured to detect a change of a charging mode for the load unit, and control to adjust an impedance in the power reception unit according to the change of the charging mode.

PRIORITY

This continuation application claims priority under 35 U.S.C. §120 toU.S. patent application Ser. No. 13/548,660, filed on Jul. 13, 2012 inthe United States Patent and Trademark Office, and is now U.S. Pat. No.9,425,629 issued on Aug. 23, 2016, which claims priority under 35 U.S.C.§119(a) to Korean Patent Applications filed in the Korean IntellectualProperty Office on Jul. 14, 2011, Jul. 15, 2011 and Jul. 11, 2012 andassigned Serial Nos. 10-2011-0070120, 10-2011-0070603 and10-2012-0075383, respectively, the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless power receiver,and more particularly, to a wireless power receiver that can maximizethe transmission efficiency of wireless power.

2. Description of the Related Art

Mobile terminals such as, for example, a mobile phone or a PersonalDigital Assistant (PDA), are driven by rechargeable batteries. Thebattery of the mobile terminal is charged through a separate chargingapparatus. In general, a separate contact terminal is arranged outsideof the charging apparatus and the battery, and the charging apparatusand the battery are electrically connected to each other through theseparate contact terminal.

However, since the contact terminal is generally protrudes outwardly ina contact type charging scheme, the contact terminal is easilycontaminated by foreign substances. Thus, battery charging is notcorrectly performed. Further, the battery charging may not be correctlyperformed when the contact terminal is exposed to moisture.

Recently, wireless charging or non-contact charging technology has beendeveloped and used for electronic devices.

Wireless charging technology employs wireless powertransmission/reception, and corresponds to, for example, a system inwhich a battery can be automatically charged if the battery is laid on acharging pad, without connecting the mobile phone to a separate chargingconnector. Wireless charging technology is generally known to be usedwith, for example, a wireless electric toothbrush or a wireless electricshaver. Accordingly, a waterproof function can be improved whenelectronic products are wirelessly charged through wireless chargingtechnology. The portability of electronic devices can be increased sincethere is no need to provide a wired charging apparatus.

Wireless charging technology largely includes an electromagneticinduction scheme using a coil, a resonance scheme using a resonance, anda Radio Frequency (RF)/microwave radiation scheme converting electricalenergy to a microwave and then transmitting the microwave.

A power transmission method through electromagnetic inductioncorresponds to a scheme of transmitting power between a first coil and asecond coil. When a magnet approaches the coil, an induced current isgenerated. A transmission side generates a magnetic field by using theinduced current, and a reception side generates energy through aninduced current according to changes in the magnetic field. Thisphenomenon is referred to as magnetic induction, and the powertransmission method using magnetic induction has a high energytransmission efficiency.

With respect to the resonance scheme, a system has been developed inwhich electricity is wirelessly transferred, using a power transmissionprinciple of the resonance scheme based on a coupled mode theory, evenwhen a device to be charged is separated from a charging device byseveral meters. A wireless charging system employs a concept that theresonance is a tendency of a wine glass to oscillate at the samefrequency as a neighboring tuning fork. An electromagnetic wavecontaining electrical energy was resonated instead of sounds. Theresonated electrical energy is directly transferred only when there is adevice having a resonance frequency. Parts of electrical energy, whichare not used, are reabsorbed into an electromagnetic field instead ofbeing spread in the air, so that the electrical energy does not affectsurrounding machines or people, unlike other electromagnetic waves.

The conventional wireless receiver changes impedance according to acharging amount, and accordingly the wireless charging efficiency isreduced. Therefore, technology is required that changes the impedance ofthe wireless power receiver based on the charging amount.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the aboveproblems and/or disadvantages and to provide at lest the advantagesdescribed below. Accordingly, an aspect of the present inventionprovides a wireless power receiver that changes impedance by itselfaccording to changes in a charging amount, and a method of controllingthe wireless power receiver.

In accordance with an aspect of the present invention, a wireless powerreceiver configured to wirelessly receive charging power from a wirelesspower transmitter is provided. The wireless power receiver includes apower reception unit configured to wirelessly receive the charging powerfrom the wireless power transmitter; a rectifier configured to rectifythe charging power from the power reception unit; a load unit configuredto store the rectified charging power from the rectifier; and acontroller configured to detect a change of a charging mode for the loadunit, and control to adjust an impedance in the power reception unitaccording to the change of the charging mode.

In accordance with another aspect of the present invention, a method forwirelessly receiving charging power at a wireless power receiver from awireless power transmitter is provided. The method includes wirelesslyreceiving the charging power from the wireless power transmitter, at awireless reception unit of the wireless power receiver; rectifying thecharging power from the power reception unit, at a rectifier of thewireless power receiver; storing the rectified charging power from therectifier, at a load unit of the wireless power receiver; detecting achange of a charging mode for the load unit; and adjusting an impedancein the power reception unit according to the change of the chargingmode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionwhen taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a wireless power transmission/receptionsystem, according to an embodiment of the present invention;

FIG. 2A is a circuit diagram illustrating a wireless power transmitterand receiver, according to an embodiment of the present invention;

FIG. 2B is a chart showing transmission efficiency of FIG. 2A, accordingto an embodiment of the present invention;

FIG. 2C is a circuit diagram illustrating a wireless power transmitterand receiver, according to an embodiment of the present invention;

FIG. 3 is a block diagram illustrating a wireless power receiver,according to embodiments of the present invention;

FIGS. 4A and 4B are circuit diagrams illustrating a wireless powertransmitters and receivers, according to embodiments of the presentinvention;

FIG. 5 is a chart showing transmission efficiency of FIGS. 4A and 4B,according to embodiments of the present invention;

FIG. 6A is a circuit diagram illustrating a wireless power transmitterand receivers, according to an embodiment of the present invention;

FIG. 6B is a chart showing transmission efficiency of FIG. 6A, accordingto a embodiment of the present invention;

FIGS. 6C and 6D are circuit diagrams illustrating wireless powertransmitter and receivers, according to embodiments of the presentinvention;

FIGS. 6E and 6F are charts showing transmission efficiency of FIGS. 6Cand 6D, according to embodiments of the present invention;

FIGS. 7A and 7B are circuit diagrams illustrating wireless powertransmitter and receivers, according to embodiments of the presentinvention;

FIGS. 7C and 7D are circuit diagrams illustrating wireless powertransmitter and receivers, according to embodiments of the presentinvention; and

FIG. 8 is a chart showing transmission efficiency of the first wirelesspower receiver, according to embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Embodiments of the present invention are described in detail withreference to the accompanying drawings. The same or similar componentsmay be designated by the same or similar reference numerals althoughthey are shown in different drawings. Detailed descriptions ofconstructions or processes known in the art may be omitted to avoidobscuring the subject matter of the present invention

FIG. 1 is a diagram illustrating a wireless power transmission/receptionsystem, according to an embodiment of the present invention. As shown inFIG. 1, the wireless power transmission/reception system includes awireless power transmitter 100 and at least one wireless power receiver110. The wireless power transmitter 100 can configure an electricalconnection with the wireless power receiver 110. In embodiments of thepresent invention, the wireless power transmitter 100 can supplywireless power in a form of an electromagnetic wave to the wirelesspower receiver 110.

The wireless power transmitter 100 can perform bidirectionalcommunication with the wireless power receiver 110. The wireless powertransmitter 100 and the wireless power receiver 110 may be apparatusesthat can process or transmit/receive a predetermined communicationpacket, and may be implemented as, for example, a mobile phone, a PDA, aPersonal Media Player (PMP), or a smart phone.

The wireless power transmitter 100 can wirelessly provide power to aplurality of wireless power receivers 110. For example, the wirelesspower transmitter 100 can transmit power to a plurality of wirelesspower receivers 110 through a resonance scheme. When the wireless powertransmitter 100 adopts the resonance scheme, it is preferable thatdistances between the wireless power transmitter 100 and the pluralityof wireless power receivers 110 be less than or equal to 30 meters (m).Further, when the wireless power transmitter 100 adopts anelectromagnetic induction scheme, it is preferable that distancesbetween the wireless power transmitter 100 and the plurality of wirelesspower receivers 110 be less than or equal to 10 m.

The wireless power receivers 110 receive wireless power from thewireless power transmitter 100 to charge batteries arranged therein.Further, the wireless power receivers 110 can transmit a signal thatrequests at least one of a wireless power transmission, informationrequired for the wireless power transmission, state information on thewireless power receiver, control information on the wireless powertransmitter 100, to the wireless power transmitter 100. Information onthe transmission signal is described in greater detail below.

Further, the wireless power receiver 110 can transmit a positioninformation message of the wireless power receiver 110. The positioninformation message of the wireless power receiver 110 may beimplemented through near field communication, such as, for example, anRF signal or Bluetooth, which are described in greater detail below.

Furthermore, the wireless power receiver 110 can transmit a chargingstate message, which indicates a state of the wireless power receiver110 to the wireless power transmitter 100.

The wireless power transmitter 100 can include a display, which displaysrespective states of wireless power receivers based on messages receivedfrom the respective wireless power receivers. Moreover, the wirelesspower transmitter 100 can display times at which the charging of therespective wireless power receivers is completed as well.

The wireless power transmitter 100 can transmit a control signal, whichdisables a charging function, to each of the wireless power receivers110. The wireless power receiver 110, having received the disablecontrol signal of the wireless charging function from the wireless powertransmitter 100, can disable the wireless charging function.

FIG. 2A is a circuit diagram illustrating a wireless power transmitterand wireless power receiver, according to an embodiment of the presentinvention. The wireless power transmitter, according to FIG. 2A,includes an amplifier 201 and a power transmission unit 211. Further,the wireless power receiver includes a power reception unit 221, arectifier 222, an internal resistance 223, an external resistance 224,and a switch unit 225. The amplifier 201 receives a supply of power,having a voltage of VDD and a current of IDD, from a power supplier. Theamplifier 201 can output the power after transmitting it in an AC at afrequency of ω₀, which may equal, for example, 6.78 megahertz (MHz). Theamplifier 201 may be embodied as a Class E Amp. The power transmissionunit 211 wirelessly transmits power to the power reception unit 221. Therectifier 222 rectifies the received power. The switch unit 225maintains an on state in a charging mode and an off state in a chargingcompletion mode. Impedance viewed from Port 2 is 10Ω when the switchunit 225 is in the on state. The impedance viewed from Port 2 is 135Ωwhen the switch unit 225 is in the off state.

As described above, a transmission efficiency S21 from the powertransmission unit 211 to the power reception unit 221 may be decreasedfrom −1 dB(231) to −8 dB(232) at 6.78 MHz, as shown in FIG. 2B.

The decrease in the transmission efficiency is described with referenceto FIG. 2C. A left side of FIG. 2C is circuit diagram illustrating thewireless power transmitter-receiver in the charging mode, according toan embodiment of the present invention. The equivalent circuit mayinclude a ground 251, a resistor 252, one end of which is connected tothe ground 251, a load 253, one end of which is connected to the otherend of the resistor 252, a load 254, one end of which is connected tothe other end of the load 253, and a resistor 255, one end of which isconnected to the other end of the load 254. Impedance of the load 253may be 50Ω, and impedance of the load 254 may be 10Ω.

A right side of FIG. 2C is a circuit diagram illustrating the wirelesspower transmitter-receiver in the charging completion mode, according toan embodiment of the present invention. It can be identified in thecharging completion mode that impedance of a load 264 is increased to135Ω. Accordingly, some of the power applied from the load 253 to theload 264 is reflected, and the total power transmission/receptionefficiency is reduced.

FIG. 3 is a block diagram illustrating a wireless power receiver,according to an embodiment of the present invention.

As shown in FIG. 3, a wireless power receiver 300 includes a powerreception unit 301, a rectifier 303, a switch unit 305, a load unit 307,an impedance adjuster (or matching unit) 309, and a controller 311.

The power reception unit 301 can wirelessly receive power from thewireless power transmitter. The power reception unit 301 may beimplemented, for example, as a loop coil having a predeterminedinductance.

The power that is input to the power reception unit 301 can be output tothe rectifier 303.

The rectifier 303 can rectify the received power. The rectifier 303 maybe implemented as a known rectifying means such as, for example, adiode. It is easily understood by those skilled in the art that there isno limitation as to how the rectifier is embodied as long as it canperform the rectification. The rectifier 303, according to an embodimentof the present invention, can be embodied as a full-bridge diode. Therectifier 303 can rectify power in an input Direct Current (DC) type topower in an AC type.

The rectifier 303 is connected to the switch unit 305, and the switchunit 305 can turn on/off the connections between the load unit 307 andthe rectifier 303.

The impedance adjuster (or matching unit) 309 may include one or more ofan inductor, a capacitor, and a resistor, and can be connected to afront end of the rectifier 303. A switch unit may be included betweenthe impedance adjuster (or matching unit) 309 and the rectifier 303. Theconnection between the impedance adjuster (or matching unit) 309 and therectifier 303 is controlled by the controller 311. For example, when theswitch unit 305 is in the off state, the controller 311 connects theimpedance adjuster (or matching unit) 309 to the rectifier 303. Theaforementioned inductor, inductance, capacitance of the capacitor, andresistance of the resistor can be changed.

Although FIG. 3 illustrates that the impedance adjuster 309 (or matchingunit) is connected to the front end of the rectifier 303, the impedanceadjuster (or matching unit) 309 may instead be connected to a rear endof the rectifier 303. In this event, the rectifier 303 may include oneor more resistors. The resistance of the resistor can be changed.

The controller 311 can control a general operation of the wireless powerreceiver 300. The controller 311 can detect the on/off state of theswitch unit 305. The controller controls the connection between theimpedance adjuster (or matching unit) 309 and the rectifier 303 based onthe on/off state of the switch unit 305. For example, the controller 311can turn a switch unit located between the rectifier 303 and theimpedance adjuster (or matching unit) 309 to an off state. Thecontroller 311 connects the impedance adjuster (or matching unit) 309 tothe rectifier 303, when the switch unit 305 remains in the off state.For example, the controller 311 turns on a switch unit located betweenthe rectifier 303 and the impedance adjuster (or matching unit) 309.

Specifically, when the wireless power receiver 300 is operated in thecharging mode, the controller 311 disconnects the impedance adjuster (ormatching unit) 309 from the rectifier 303. Further, when the wirelesspower receiver 300 is operated in the charging completion mode, thecontroller 311 connects the impedance adjuster (or matching unit) 309 tothe rectifier 303. Here, the charging mode may be a Constant Current(CC) mode, and the charging completion mode may be a Constant Voltage(CV) mode.

In the CC mode, the impedance adjuster (or matching unit) 309 canperform impedance matching. Accordingly, the impedance adjuster (ormatching unit) 309 reduces the increased impedance in the chargingcompletion mode and thus converts the impedance to impedance in thecharging mode. For example, the impedance adjuster 309 can reduce theimpedance of the load 264 of FIG. 2C to 10Ω, and accordingly the powertransmission reflection can be reduced.

A regulator can be disposed between the rectifier 303 and the load unit307. The regulator can filter ripples from input rectified wirelesspower, and then output the filtered wireless power. The regulator may beimplemented as an LC filter in an embodiment of the present invention,and accordingly, compensate so that the rectified wireless power iscloser to an AC waveform. Further, the regulator can control an outputof the wireless power so that an overflow is not generated when thewireless power is output through an output terminal. The wireless poweroutput by the regulator is output externally, and then may be applied toa load or stored in the load unit 307.

FIGS. 4A and 4B are circuit diagrams illustrating implementation of thewireless power receiver, according to an embodiment of the presentinvention.

As shown in FIGS. 4A and 4B, a wireless power transmitter includes anamplifier 401 and a power transmission unit 411. Further, a wirelesspower receiver includes a power reception unit 421, a rectifier 422, aninternal resistor 423, an external resistor 424, a first switch unit425, a controller 426, a second switch unit 427, an inductor unit 428, acapacitor unit 429, and a ground unit 430.

The amplifier 401 receives a supply of power having a voltage of VDD anda current of IDD from a power supplier. The amplifier 401 outputs thepower after transmitting it in an AC at a frequency of ω₀, which may be,for example, 6.78 MHz. The amplifier 401 may be embodied as a Class EAmp. The power transmission unit 411 wirelessly transmits power to thepower reception unit 421. The rectifier 422 rectifies the receivedpower. Meanwhile, the first switch unit 425 maintains an on state in thecharging mode and an off state in the charging completion mode. Theimpedance viewed from Port 2 is 10Ω when the first switch unit 425 is inthe on state, and the impedance viewed from Port 2 is 135Ω when thefirst switch unit 425 is in the off state. In FIG. 4A, the second switchunit 427 maintains the off state. For example, the controller 426maintains the second switch unit 427 in the off state by detecting thatthe first switch unit 425 is in the on state.

In FIG. 4B, the controller 426 can detect the charging completion modeby identifying that the first switch unit 425 is in the off state. Then,the controller 426 can switch the second switch unit 427 to the onstate. When the second switch unit 427 is in the on state, the inductorunit 428, the capacitor unit 429, and the ground unit 430 can beconnected to the rectifier 422. Accordingly, impedance viewed from Port2 can be adjusted to 10Ω. Here, the combined impedance of the inductorunit 428 and the capacitor unit 429 may be greater than or equal to theimpedance viewed from Port 2 in the charging mode, for example, 10Ω, andless than the impedance viewed from Port 2 in the charging completionmode, for example, 135Ω.

A resistor unit can be provided with the inductor unit 428 and thecapacitor unit 429. Further, as described above, a switch and a resistorcan be connected in parallel to a rear end of the rectifier 422. Whenthe second switch unit 427 is in the on state, the impedance can beadjusted by the resistor.

Specifically, as described above, the impedance increase in the chargingcompletion mode or the CV mode can be reduced based on the impedanceadjuster. Accordingly, as shown in FIG. 5, the transmission efficiencyS21 can be reduced from −1 dB to −3.2 dB. Specifically, it can beidentified that the problem related to the transmission efficiencydecrease is relieved through a comparison with the transmissionefficiency of −8 dB in FIG. 2B.

FIG. 6A is a circuit diagram illustrating a wireless power transmitterand a wireless power receiver, according to an embodiment of the presentinvention. The wireless power transmitter, according to FIG. 6A,includes the amplifier 201 and the power transmission unit 211. Further,the first wireless power receiver includes the power reception unit 221,the rectifier 222, the internal resistor 223, the external resistor 224,and the switch unit 225. A second wireless power receiver includes asecond power reception unit 251, a second rectifier 252, a secondinternal resistor 253, a second external resistor 254, and a secondswitch unit 255.

The amplifier 201 receives a supply of power, having a voltage of VDDand a current of IDD, from a power supplier, and can output the powerafter transmitting it in an AC at a frequency of ω₀, such as 6.78 MHz.The amplifier 201 may be embodied as a Class E Amp. The powertransmission unit 211 wirelessly transmits power to the power receptionunit 221. The rectifier 222 rectifies the received power. The switchunit 225 maintains an on state in a charging mode and an off state in acharging completion mode. Impedance viewed from Port 2 is 10Ω when theswitch unit 225 is in the on state, and the impedance viewed from Port 2is 135Ω when the switch unit 225 is in the off state.

The second power reception unit 251 also wirelessly receives power fromthe power transmission unit 211. The second rectifier 252 rectifies thereceived power. The second switch unit maintains the on state in thecharging mode and the off state in the charging completion mode.

It is assumed herein that the first wireless power receiver is in thecharging mode and the second wireless power receiver is shifted to thecharging completion mode.

When the second wireless power receiver's own impedance is changed sincethe second wireless power receiver has been shifted to the chargingcompletion mode, the impedance of the first wireless power receiver maybe affected. Further, the change of the impedance of the second wirelesspower receiver also affects the transmission efficiency S21 of the firstwireless power receiver.

FIG. 6B is a graph of the transmission efficiency S21 of the firstwireless power receiver when the second wireless power receiver's ownimpedance is changed. In FIG. 6B, it can be identified that thetransmission efficiency S21 is reduced from −4.5 Db(601) to −8 dB(602)at 6.78 MHz. Specifically, the change of the second wireless powerreceiver's own impedance affects the transmission efficiency S21 of thefirst wireless power receiver. The aforementioned phenomenon isdescribed with reference to FIG. 6C below.

FIG. 6C is a circuit diagram illustrating the wireless powertransmitter, the first receiver, and the second receiver, according toan embodiment of the present invention. The circuit includes a firstresistor 611, one end of which is grounded, a first load 612, one end ofwhich is connected to the other end of the first resistor 611, a secondload 613, one end of which is connected to the other end of the firstload 612, and a third load 614, one end of which is connected to theother end of the first load 612. Further, a fourth load 615 can beformed between the other end of the second load 613 and the other end ofthe third load 614. The fourth load 615 can be formed by a couplingbetween the first wireless power receiver and the second wireless powerreceiver.

One end of a second resistor 616 is connected to the second load 613,and the other end of the second resistor 616 is connected to a firstground 617. Further, one end of a third resistor 618 is connected to thethird load 614 and the other end of the third resistor 618 is connectedto a second ground 619. In the above embodiment of the presentinvention, it is assumed that the first wireless power receiver isoperated in the charging mode and the second wireless power receiver isoperated in the charging completion mode. The first load 612 may be 50Ω,the second load 613 may be 10Ω, the third load 614 may be 135Ω, and thefourth load 615 may be 50Ω.

FIG. 6D is a circuit diagram illustrating, the wireless powertransmitter, the first receiver and the second receiver, according to anembodiment of the present invention. In FIG. 6D, one end of a firstresistor 632 is connected to a first ground 631, and the other end ofthe first resistor 632 is connected to one end of a first load 633. Theother end of the first load 633 is connected to one end of a second load634. The other end of the second load 634 is connected to one end of athird load 635 and one end of a fourth load 636. The other end of thethird load 635 is connected to one end of a second resistor 637, and theother end of the second resistor 637 is connected to a second ground638. The other end of the fourth load 636 is connected to one end of athird resistor 639, and the other end of the third resistor 639 isconnected to a third ground 640. The load first 633 may be 50Ω, thesecond load 634 may be 6.92Ω, the third load 635 may be 346Ω, and thefourth load 636 may be 2.56Ω.

The loads of FIG. 6C and the loads of FIG. 6D are related as shown inEquations (1) to (3) below.

$\begin{matrix}{Z_{1}^{\prime} = \frac{Z_{2}Z_{3}}{Z_{1} + Z_{2} + Z_{3}}} & (1) \\{Z_{2}^{\prime} = \frac{Z_{1}Z_{3}}{Z_{1} + Z_{2} + Z_{3}}} & (2) \\{Z_{3}^{\prime} = \frac{Z_{1}Z_{2}}{Z_{1} + Z_{2} + Z_{3}}} & (3)\end{matrix}$

In Equations (1) to (3), Z₁, Z₂, and Z₃ are the impedance of the secondload 613, the third load 614, and the fourth load 615 of FIG. 6C,respectively. Further, Z′₁, Z′₂, and Z′₃ are the impedance of the secondload 634, the third load 635, and the fourth load 636 of FIG. 6D,respectively.

Meanwhile, as shown in FIG. 6D, the impedance of the third load 635 isincreased more in comparison with the impedance of the second load 613of FIG. 6C, and accordingly the transmission efficiency S21 of the firstwireless power receiver is reduced.

FIGS. 6E and 6F are diagrams illustrating transmission efficiencies ofthe first wireless power receiver corresponding to a case where thesecond wireless power receiver is in the charging mode and the secondwireless power receiver is in the charging completion mode, according toan embodiment of the present invention. As shown in FIGS. 6E and 6F,when the second wireless power receiver's own impedance is changed, thetransmission efficiency of the first wireless power receiver is changed.

Accordingly, it is desirable to develop a technology in which thetransmission efficiency of the first wireless power receiver is notaffected even when the second wireless power receiver's impedance ischanged.

FIGS. 7A and 7B are circuit diagrams illustrating a wireless powertransmitter, a first wireless power receiver, and a second wirelesspower receiver, according to embodiments of the present invention.

As shown in FIGS. 7A and 7B, a wireless power transmitter includes anamplifier 711 and a power transmission unit 912. A first wireless powerreceiver includes a first power reception unit 721, a first rectifier722, a first internal resistor 723, a first external resistor 724, afirst switch unit 725, a controller 726, a second switch unit 727, aninductor unit 728, a capacitor unit 729, and a first ground unit 730. Asecond wireless power receiver includes a second power reception unit751, a second rectifier 752, a second internal resistor 724, a secondexternal resistor 756, and a third switch unit 753.

In an embodiment of the present invention shown in FIG. 7A, it isassumed that both the first and second wireless power receivers are inthe charging mode.

The amplifier 711 receives a supply of power, having a voltage of VDDand a current of IDD, from a power supplier. The amplifier can outputthe power after transmitting it in an AC having a frequency of ω₀, suchas 6.78 MHz. The amplifier 711 may be embodied as a

Class E Amp. The power transmission unit 712 wirelessly transmits powerto the first power reception unit 721 and the second power receptionunit 751. The first and second rectifiers 722 and 752 rectify thereceived power.

The first and third switch units 725 and 753 maintain the on state inthe charging mode and the off state in the charging completion mode.Impedance viewed from Port 2 is 10Ω when the first and third switchunits 725 and 753 are in the on state, and the impedance viewed fromPort 2 is 135Ω when the first and third switch units 725 and 753 are inthe off state.

In FIG. 7A, the second switch unit 727 maintains the off state. Forexample, the controller 726 maintains the second switch unit 727 in theoff state by detecting the on state of the first switch unit 725.

In FIG. 7B, the controller 726 can detect the charging completion modeby identifying that the first switch unit 725 is in the off state. Thecontroller 726 can switch the second switch unit 727 to the on state.When the second switch unit 727 is in the on state, the inductor unit728, the capacitor unit 729, and the ground 730 can be connected to therectifier 722. Accordingly, impedance viewed from Port 2 can be adjustedto 10Ω. Also, as the first wireless power receiver's own impedance isreadjusted, the transmission efficiency of the second wireless powerreceiver is not affected.

FIGS. 7C and 7D illustrate equivalent circuits of FIG. 7B.

The equivalent circuit of FIG. 7C includes a first resistor 772, one endof which is connected to a first ground 771, a first load 773, one endof which is connected to the other end of the first resistor 772, asecond load 774, one end of which is connected to the other end of thefirst load 773, and a third load 775, one end of which is connected tothe other end of the first load 773. Further, a fourth load 776 can beformed between the other end of the second load 774 and the other end ofthe third load 775. The fourth load 776 can be formed by a couplingbetween the first wireless power receiver and the second wireless powerreceiver.

One end of a second resistor 777 is connected to the second load 774,and the other end of the second resistor 777 is connected to a secondground 778. Further, one end of a third resistor 779 is connected to thethird load 775, and the other end of the third resistor 779 is connectedto a third ground 760. In the above embodiment of the present invention,it is assumed that the first wireless power receiver is operated in thecharging completion mode, the second wireless power receiver is operatedin the charging mode, and the impedance adjuster is connected. The firstload 773 may be 50Ω, the second load 774 may be 10Ω, the third load 775may be 10Ω, and the fourth load 776 may be 50Ω. Specifically, theimpedance of the third load 775 can be changed to 10Ω by the impedanceadjuster.

FIG. 7D illustrates an equivalent circuit of FIG. 7C, and can be formedthrough Equations (1) to (3) described above. In FIG. 7D, one end of afirst resistor 782 is connected to a first ground 781, and the other endof the first resistor 782 is connected to one end of a first load 783.One end of a second load 784 is connected to the other end of the firstload 783. One end of a third load 785 and one end of a fourth load 788are connected to the other end of the second load 784. One end of asecond resistor 786 is connected to the other end of the third load 785,and the other end of the second resistor 786 is connected to a secondground 787. One end of a third resistor 789 is connected to the otherend of a fourth load 788, and the other end of the third resistor 789 isconnected to a third ground 770. Here, the first load 783 may be 50Ω,the second load 784 may be 1.42Ω, the third load 785 may be 7.1Ω, andthe fourth load 788 may be 7.1Ω. The first load 785 and the fourth load788 have the same impedance, and accordingly a problem in which thetransmission efficiency of the first wireless power receiver is reducedcan be relieved.

FIG. 8 is a diagram illustrating the transmission efficiency of thefirst wireless power receiver according to embodiments of the presentinvention. As shown in FIG. 8, it can be identified that thetransmission efficiency of the first wireless power receiver is onlyslightly affected although internal impedance of the first wirelesspower receiver is changed, the transmission efficiency being changedfrom −1 dB to 1.5 dB.

While the invention has been shown and described with reference tocertain embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A wireless power receiver configured towirelessly receive charging power from a wireless power transmitter, thewireless power receiver comprising: a power reception unit configured towirelessly receive the charging power from the wireless powertransmitter; a rectifier configured to rectify the charging power fromthe power reception unit; a load unit configured to store the rectifiedcharging power from the rectifier; and a controller configured to:detect a change of a charging mode for the load unit, and control toadjust an impedance in the power reception unit according to the changeof the charging mode.
 2. The wireless power receiver as claimed in claim1, wherein the controller detects that the charging mode is changed froma Constant Current (CC) mode to a Constant Voltage (CV) mode.
 3. Thewireless power receiver as claimed in claim 2, further comprising animpedance adjuster configured to adjust an impedance in the powerreception unit, and wherein, the controller is further configured toconnect the impedance adjuster to at least one of a front end and a rearend of the rectifier in response to detecting the change of the chargingmode.
 4. The wireless power receiver as claimed in claim 3, furthercomprising one or more switch units arranged between the impedanceadjuster and the at least one of the front end and the rear end of therectifier, wherein the controller is further configured to control theone or more switch units to be in an off state in the CC mode and to bein an on state in the CV mode.
 5. The wireless power receiver as claimedin claim 3, wherein the impedance adjuster is connected to the front endof the rectifier in the CV mode.
 6. The wireless power receiver asclaimed in claim 5, wherein the impedance adjuster comprises one or moreof a capacitor, a coil, and a resistor.
 7. The wireless power receiveras claimed in claim 6, wherein the capacitor, the coil, and the resistorare a variable capacitor, a variable coil, and a variable resistor,respectively.
 8. The wireless power receiver as claimed in claim 3,wherein the impedance adjuster is connected to the rear end of therectifier in the CV mode.
 9. The wireless power receiver as claimed inclaim 7, wherein the impedance adjuster comprises one or more resistors.10. The wireless power receiver as claimed in claim 9, wherein the oneor more resistors are variable resistors.
 11. The wireless powerreceiver as claimed in claim 3, wherein the impedance adjuster isfurther configured to adjust the impedance such that the impedance ofthe wireless power receiver is substantially equal in the CC mode andthe CV mode.
 12. A method for wirelessly receiving charging power at awireless power receiver from a wireless power transmitter, the methodcomprising the steps of: wirelessly receiving the charging power fromthe wireless power transmitter, at a wireless reception unit of thewireless power receiver; rectifying the charging power from the powerreception unit, at a rectifier of the wireless power receiver; storingthe rectified charging power from the rectifier, at a load unit of thewireless power receiver; detecting a change of a charging mode for theload unit; and adjusting an impedance in the power reception unitaccording to the change of the charging mode.
 13. The method as claimedin claim 12, wherein detecting the change of the charging mode comprisesdetecting that the charging mode is changed from a Constant Current (CC)mode to a Constant Voltage (CV) mode.